The merge of recent high-speed telecommunications demands wider and wider signal bandwidth for use in these telecommunications systems. However, while attempting to widen the available bandwidth of channels within a given transmission frequency range, for example, in 5 GHz technologies, the combined use of amplitude and phase modulation, such as may be found in orthogonal frequency division multiplex (OFDM) signals, has proven difficult to implement. More specifically, the delivery of improved spectral efficiency of transmitted signals in such so-called linear modulation schemes typically undergoes significant distortion (both phase and amplitude) when the modulated signals are boosted by a power amplifier for transmission to a receiver. Such distortion is especially prevalent in transmitters that implement power efficient, but nonlinear, power amplifiers. As a result, linearization techniques have been developed to produce a desirable trade-off between a transmitter's efficiency and its linearity.
Among the more popular linearization techniques developed to combat this problem is Cartesian feedback linearization. In this type of linearization, a Cartesian feedback is provided after the power amplifier has introduced undesirable distortion into a modulated output signal. The Cartesian feedback is used to compare baseband in-phase and quadrature phase input signals with the distorted baseband in-phase and quadrature phase feedback signals demodulated from the power amplifier. The loop then introduces “pre-distortion” into the modulated signal prior to the power amplifier so that distortion introduced by the power amplifier is offset to achieve the desired linearization.
Unfortunately, Cartesian feedback linearization has typically only proved successful with narrowband systems. This is this case since broadband systems implement channels having large bandwidth differentials therein. The more delay or phase distortion a power amplifier introduces into the modulated output signal, the more steep the phase versus frequency curve corresponding to a given bandwidth becomes. Thus, if a large bandwidth differential between multiple channels exists, which is typically the case in broadband systems, the Cartesian feedback loop may pre-distort one channel but not other channels having a large bandwidth differential from the first. As a result, power amplifiers having specifically designed phase characteristics for predetermined bandwidth differentials are usually required to provide the linearization necessary to avoid oscillations at in- and out-of-band frequencies in the circuit loop of a broadband transmitter that may occur at frequencies where large amplitude and phase distortions are present. However, complex design and high manufacturing costs of such specialized power amplifiers have driven manufacturers to seek out a better solution.
Accordingly, what is needed in the art is an RF transmitter for use in broadband communications systems that does not suffer from the deficiencies found in the prior art.